The statements in this section merely provide background information related to the present invention and may not necessarily constitute prior art.
In a motor vehicle, a coupling in the powertrain serves for changing the transfer distribution of the driving torque to the different axles of the motor vehicle and/or to the different wheels of a respective axle. Sufficient traction should hereby be ensured in all imaginable driving situations, in particular when driving off-road. The respective coupling is associated with a transfer case for this purpose, for example. The transfer case can be operative in the longitudinal direction of the vehicle, i.e. between different axles. The powertrain can, for example, have an inter-axle differential gear which distributes the driving torque permanently onto the two associated axles and which is selectively locked by means of the coupling (so-called longitudinal differential lock). Alternatively to this, it can be a case of a transfer case of the “torque-on-demand” type in which a primary axle is constantly driven and a secondary axle is selectively activated, i.e. additionally driven, by means of the coupling. The coupling can also be associated with an axle differential gear which is operative in the transverse direction, which distributes the driving torque to the wheels of the respective axle and which is selectively locked by means of the coupling (so-called transverse differential lock or axle differential lock). Finally, the coupling can also be associated with a wheel drive of the powertrain which can be activated, i.e. the driving torque can selectively be transferred to a single wheel by means of the coupling.
The motor vehicle can have two or more axles and accordingly a plurality of couplings of the named kind can be provided, for example one or more longitudinal differential locks and two or more transverse differential locks. If the actuation of a plurality of couplings in the powertrain should take place in an automated manner as much as possible, the control of the couplings becomes very complex.
Special demands have to be made in this respect if it is an off-road vehicle, in particular an off-road vehicle having more than two axles. It is particularly important in an off-road vehicle that a loss of traction is avoided at an early time and effectively since it can otherwise be very difficult in unfavorable driving situations to return to a stable driving situation (for example with a small or greatly varying coefficient of friction of the ground, of a slope and/or of a side position). Whereas the respective coupling can generally be made as a multi-disk coupling operative with friction engagement, with all-terrain vehicles form-fitting couplings, i.e. dog couplings, are typically used to be able to transfer high torques with low wear.
The actuation of the respective coupling takes place as a consequence of a corresponding control by means of a suitable actuator, for example pneumatically, hydraulically, electromechanically or electromagnetically. To produce a corresponding control signal for the respective coupling, slip values can be determined and evaluated which represent an instantaneous slip between different axles of the motor vehicle or between different wheels of an axle. It is therefore in the widest sense a case of speed of rotation differences or speed differences, for example, the speed of rotation difference between the two output shafts of a differential gear or the difference between the wheel speed signals of an axle. For example, the respective slip value or a value derived herefrom can be compared with a suitable threshold value to generate a control signal for actuating the coupling in the case of an exceeding of the threshold value.
Provided the determined slip values are compared directly with a threshold value, there is, however, the risk of too early a shift of the coupling, for example when a comparatively large slip should only be detected for a short period, but which can also result only from the elasticity of the powertrain. To avoid this problem, it is possible to add the determined speed of rotation differences to form slip sum values which are compared with a slip sum threshold value. Such a process is known from the document EP 0 644 077 B1 in which the control of the coupling should take place very fast in the event of a rapidly increasing speed of rotation difference, but only after a longer period with a small speed of rotation difference. An evaluation independent of the sign should take place by the formation of slip sums here in the event of changing slip (leading slip and trailing slip) to avoid unnecessary shifts. This document also teaches providing a hierarchical structure for a plurality of couplings of a powertrain, with couplings lower in the hierarchy also outputting control signals for couplings higher in the hierarchy. A disadvantage of the process described in the document EP 0 644 077 B1 is that slip oscillations, i.e. repeated variations in the sign of the slip occurring within a limited time window are not sufficiently taken into account. In addition, the required calculation steps are undesirably complex.
In the document EP 1 468 860 B1 an addition of slip values also takes place up to the reaching of a predefined threshold value, with the taking into account of leading slip and trailing slip being carried out separately, i.e. the calculations and evaluations are carried out separately for the different signs of the determined slip values. A disadvantage of this process is in turn the complex manner of calculation since the calculations have to be carried out twice in principle, namely both for leading slip and for trailing slip. An evaluation of the determined slip values which is as fast as possible and a corresponding generation of control signals is, however, of great importance particularly with couplings operative in form-fitting manner in the powertrain of an off-road vehicle since otherwise speed of rotation differences arise which make it impossible to close the respective coupling in time.